Pathogenic microbes are exposed to iron limiting conditions and iron-catalized oxidative damage and therefore must possess virulence factors that antagonize these stresses. Because iron storage proteins store up to 4,500 iron atoms in a non-toxic form, we have hypothesized that they help pathogens to withstand iron deprivation and protect against oxygen and nitrogen dependent (i.e., the NADPH oxidase-dependent respiratory burst and nitric oxide synthesis) antimicrobial systems of phagoytic cells. We specifically propose to construct S.typhimurium strains deficient in the iron storage proteins ferritin and bacterioferritin. These strains will be tested for resistance to iron deprivation, and oxidative and nitrosative stress in macrophages and mice; the mutants will be further characterized by examining for dithiol -nitrosyl-iron complexes after exposure to NO donors and activated macrophages. These studies will not only produce novel insights into complex host-pathogens interactions in an important opportunistic infection but will also provide new information on the chemical interactions of different NO species with non-heme iron proteins.